This invention relates to a feeding and a mixing or blending apparatus and method, and more particularly to an apparatus and method for automatically and continuously blending and mixing quantities of particulate material of a granulated, powdered, or pelletized quality. At the outset it should be appreciated though that while the invention is described as applied to a mixing or blending apparatus, it is equally advantageous for use as a pure feeding mechanism exclusive of any mixing or blending functions.
In the production of finished molded plastic products from injection, blow and extrusion molding and like procedures, it is common to mix different types of raw materials tegether in predetermined quantities to produce a proper blend for the molding process. The polymeric materials generally employed will be in the form of dry particulate material of a granulated, powdered, or pelletized quality, the final blend being typically a combination of these various raw materials. For example, in a typical injection molding process, several different types of plastic in the form of virgin polymer, a reground polymeric material recovered from scrap, and a colorant or like additive are mixed together in a fixed ratio before being fed to an injection molding apparatus. The ratio of the materials making up the final feed blend is often of critical importance, since the quality and consistency of the finished product can be altered with even minor variances in the constituent materials making up the feed product. Raw materials must thus be precisely measured out in the correct amounts, and homogeneously mixed and blended to yield a feed which will maintain the desired ratio throughout the course of a continuous production run.
Where batch mixing is employed, the ability to make a homogeneous mix is of no particular difficulty, since thorough mixing of the combined materials is usually effected before the batch is progressed through the system. When using a continuous production operation, however, it is often difficult to precisely blend the various raw materials before the feed passes through the mixing area to a take-off which in turn communicates with and delivers the final product feed to the molding apparatus. Unless properly blended, undesirable concentrations of several constituent materials may be maintained in the final product feed, thus yielding a non-uniform finished product. The accurate and thorough mixing and blending of the disparate materials making up the final feed is thus a highly desired feature for any continuous mixing apparatus.
Maintaining the proper ratio of raw materials in the product feed is often difficult. Many prior art devices employ systems which entail the addition of a separate and additional metering apparatus for each material which is to be added to the final product feed, each metered product being delivered to a central chamber for mixing together. Each additional metering system adds an additional factor of potential error and required control into the mixing of the final feed. This becomes more important where one of the constituents making up the final feed predominates, which is the usual case. Metering of the predominant quantity can entail the use of a larger metering device capable of handling the larger quantity, or a faster feed rate in smaller metering equipment with a concomitant greater potential for error in producing the requisite volume of material for the final feed blend. Further, such equipment often uses a force feed principle for metering out the desired quantities of raw materials. These force feed processes typically pneumatic or vacuum induced, further degrade the accuracy and control of the final feed blend; the mechanical movement of the disparate materials is undesirable, since such automatic metering mechanisms will operate regardless of the amount of material actually present in the system, the raw material being fed being typically subject to blockages, unequal flow rates, or other discontinuities of flow which will not be detected by the automatic force feed devices.
Some prior art devices have attempted to overcome these difficulties in part by applying a gravity feed to the major constituent of the feed product. The major or primary raw material is generally deposited into a hopper/mixing chamber combination, additives being metered into the chamber for mixing with the primary material. Mixing of the various raw materials within the hopper/mixing chamber may occur through use of a stirring mechanism disposed within the hopper/mixing chamber, or within a plasticizing or take-off screw which ordinarily is in communication with the end port of the chamber. It should be noted that mixing and blending effected only by a plasticizing or take-off screw is often undesirable, since a localized concentration of a raw material may not be removed in the course of the screw feed, thus producing a non-homogeneous feed for the final product.
The hopper/mixing chamber device, although eliminating a major metering operation, nevertheless suffers from several infirmities. First, the material being directly added to the hopper/mixing chamber is subject to bridging and rat-holing, which will affect the regular flow of material through the take-off screw. Further, the flow rate of the screw can be affected by the pressure and load factors presented by the raw material in the hopper. The amount and bottom density of material in the hopper will thus affect the amount of material being drawn off by the take-off screw. That is, more material in the hopper will increase the pressure on, and thus the density of, the material at the withdrawal point of the screw. The flow of material from the take-off screw may thus significantly vary depending on the amount of material in the hopper.
Given that it is highly desirable to carry out mixing and accurately supplying the raw materials on an automated and continuous basis to achieve a maximum production rate with the least amount of manual intervention and control, it can be seen that there is a need in the industry for a device which can reduce the amount of direct metering of raw materials by gravity feeding the primary constituent, but do it in such a way that a constant blend of materials is maintained at a constant flow rate that is not affected by the amount of raw material in the primary hopper.
An object of the instant invention is therefore to provide an apparatus and method which are capable of automatically, accurately and continuously combining a primary material, such as a particulate virgin feed, with an additive material, such as a colorant and/or regrind particulate scrap, where the combined materials are potentially of different physical character, and to do so with an unmetered gravity feed of the primary raw material directly to the mixing chamber in such a manner as to insure the constant flow of feed with the proper ratio of blended raw materials to a discharge mechanism, the load imposed on the discharge mechanism and the density of material at the discharge point remaining relatively constant.
Another object is to provide an apparatus and method which are capable of continuously feeding one or more particulate materials to a discharge mechanism in such a manner as to insure a constant density of a given material or materials at the discharge mechanism, as well as a constant load on the discharge mechanism.